A method for producing variable electron beam spot sizes and shapes has been described by H. C. Pfeiffer in an article entitled "Variable Spot Shaping for Electron-Beam Lithography"--Journal of Vacuum Science Technology, Volume 15, 887 (1978) and to the application of such techniques to electron beam lithography. With respect to resolution and overlay accuracy, electron beam lithography is a superior method of writing integrated circuit patterns either directly on a semiconductor wafer or indirectly on glass or membranes for use as optical or x-ray masks. However, the use of electron lithography for direct write production of integrated circuits on a large commercial scale is limited by the relatively low rate at which features can be written compared to optical, or possibly in the future, x-ray lithography. The basic reason for this comparative speed disadvantages lies in the serial nature of writing pattern pixels with electron beams while optical patterning through a previously written mask is basically a parallel writing operation with many pixels written at one time.
In the extreme case of a finely focused Gaussian spot, an array of four by four Gaussian spots are typically required to generate the smallest feature of a desired electron beam pattern. This requirement arises due to the sharp edge width which is required in writing integrated circuit patterns.
An important improvement in speed of electron beam writing was made by H. C. Pfeiffer as reported in the above-noted 1978 article and which introduced a method of forming variable, different size electron beam spots with sharp edges and containing one or more pattern pixels.
A second step in improving the writing speed of electron beam lithography systems has been described in U.S. Pat. No. 4,390,789 for "Electron Beam Array Lithography System Employing Multiple Parallel Array Optic Channels", inventors--Donald O. Smith and Kenneth J. Harte and U.S. Pat. No. 4,694,178 for "Multiple Channel Electron Beam Optical Column Lithography System and Method of Operation", inventor--Kenneth J. Harte. In these prior patents, multiple electron beams are provided in the form of a multiplicity of separate, miniature electron beam optical columns arranged in configurations which increase the net writing speed on a workpiece by factors of ten to twenty. In order to be able to combine the advantage of variable spot shaping and multiple miniature electron beam optical columns, a miniaturized electrostatic spot shaper has been described in U.S. Pat. No. 4,683,366 for "All Electrostatic Electron Optical Sub-System for Variable Spot Shaping and Method of Operation", inventors--Kenneth J. Harte and Guenther O. Langner.
An important improvement in the performance of such miniature electron beam columns can be obtained by using photo-cathodes as described in U.S. Pat. No. 4,460,831 for "Laser Stimulated High Current Density Photo-Cathode Generator and Method of Manufacture", inventors--P. E. Oettinger and C. Lee, and in U.S. Pat. No. 4,820,927 issued Apr. 11, 1989 for "Electron Beam Source Employing a Photo-Emitter Cathode", inventors--Guenther O. Langner and Kenneth J. Harte. In these prior patents the use of a photo-emitting cathode excited by a diode laser light or a light emitting diode is described. The advantages of using such a photo-emitting cathode lies in the details of electron optical design and is described in these patents. In these prior art patents, however, a single diode laser or light emitting diode is used to excite each separate photo-emitter cathode.